1. Field of the Invention
The present invention relates to a hydrogen storage material capable of reversibly storing and releasing hydrogen, and a method for producing the same.
2. Description of the Related Art
As is known in the art, fuel cells generate electricity using a fuel gas such as hydrogen supplied to an anode and an oxidant gas such as oxygen supplied to a cathode. Therefore, for example, a fuel-cell car utilizing the fuel cell has a vessel for storing the hydrogen gas. The oxidant gas of the air and the hydrogen gas supplied from the gas storage vessel are used as reactant gases to drive the fuel-cell car.
As is clear from the above, as the gas storage vessel has a higher hydrogen storage capacity, the fuel-cell car can be driven over a longer distance. However, when the fuel-cell car has an excessively large vessel, the weight of the fuel-cell car is increased, resulting in a higher load on the fuel cell disadvantageously.
From this viewpoint, various studies have been made on a gas storage vessel having a high hydrogen storage capacity with a small volume. For example, in Japanese Laid-Open Patent Publication No. 2004-018980, AlH3, one of hydrogen storage materials capable of reversibly storing and releasing hydrogen, is used to increase the hydrogen storage capacity.
A crystalline AlH3 1 shown in FIG. 10 has a microstructure containing matrix phases 2 approximated by squares and a grain boundary phase 3 disposed between the matrix phases 2, 2. In this case, the matrix phases 2 have a side length t1 of approximately 100 μm, and the grain boundary phase 3 has a width w1 of several micrometers and occupies only a small volume percentage in the structure. When the crystalline AlH3 is subjected to an X-ray diffraction measurement, sharp peaks of at least one of α, β, and γ phases can be found in the diffraction pattern.
It should be noted that the matrix phases are composed of AlH3 having a crystal lattice containing Al and H, while the grain boundary phase is composed of a solid solution of H in an amorphous Al.
As a result of intense research, the inventor has found that, in the process of storing hydrogen in the crystalline AlH3, the grain boundary phase 3 firstly contributes to the hydrogen storage. The grain boundary phase 3 can store hydrogen even at relatively low pressure. However, since the ratio of the grain boundary phase 3 to the structure is only several % by volume as described above, the hydrogen storage in the grain boundary phase 3 rapidly reaches the limit. It should be understood that the grain boundary phase 3 has a remarkably low hydrogen storage capacity.
Then, the matrix phases 2, which occupy the large part in the structure, begin to store hydrogen. However, significantly high activation energy is needed for the hydrogen storage in the matrix phases 2. The matrix phases 2 cannot store hydrogen at low hydrogen pressure if the temperature is constant. When the hydrogen pressure reaches several thousand atm (several hundred MPa), the matrix phases 2 can actively store hydrogen.
However, the fill pressure of the gas storage vessel is generally around 20 MPa, maximally around 75 MPa. Therefore, when the crystalline AlH3 is used in the gas storage vessel, it is difficult to store hydrogen in the matrix phases 2.
As described in Japanese Laid-Open Patent Publication No. 2004-018980, the AlH3 begins to release hydrogen at about 130° C., and the stored hydrogen is completely released before the temperature reaches 200° C. Therefore, in the case of using the gas storage vessel containing the crystalline AlH3 in the fuel-cell car, in order to supply a large amount of hydrogen from the vessel to the fuel cell, it is necessary to apply heat until the vessel has a temperature of 130° C. or higher. Thus, the amount of heat supplied to other components in the fuel-cell car is reduced, whereby it is difficult to increase the heat efficiency of the entire system of the fuel-cell car.